In general, electrostatographic printing machines have control systems that insure print appearance throughout the operating life of a machine. Xerographic process control systems are typically embedded systems with interactive feedback loops that are absent operator adjustments. These embedded control systems compensate for photoreceptor dark decay, laser or flash lamp aging, etc. In addition, embedded control systems are self-adjusting so as to insure that print engine settings are invariant from one system to another. The invariant nature of the print or process control settings insures consistent image quality from one machine to another. This lack of operator control over image quality during print runs insures that systems operate within a preset operating range. Color printing systems with stringent image quality requirements have xerographic subsystems that are sensitive to use as well as environmental changes.
Xerographic process feedback control systems are well known. Such feedback systems include systems for charging control, exposure and illumination control, developer control, toner dispensing control, etc. For example, U.S. Pat. No. 4,348,099 discloses the use of test patches, an infrared densitometer, and an electrometer for charge, illumination, toner dispenser, and developer bias control. Other examples of feedback control systems are disclosed in U.S. Pat. Nos. 5,416,564; 5,006,892; 5,225,837; 4,348,099; 5,081,491; and 5,081,492. A further example of process control is U.S. Pat. No. 5,132,730 which discloses a single pass, tri-level, printing machine with a first electrostatic voltmeter (ESV) that reads the voltage level of a color image prior to development and a second ESV after development. The difference between these two readings is compared to an arbitrary target value and a machine cycle down is initiated if the difference is greater than the target. Other process control systems using two ESVs are disclosed in U.S. Pat. Nos. 5,138,378; 5,157,441; 5,223,897; 5,227,270; 5,132,730 and 5,285,241. Process controls, which are for example present in the 5775 Digital Color Copier manufactured by the Xerox.RTM. Corporation, are necessary to maintain stringent image quality or print appearance requirements.
In contrast, lithography or offset printing offer greater control over image quality to a machine operator during print runs. The offset printing paradigm provides highly skilled operators with control over the process of an offset printing press. For example, such operators are provided with the ability to adjust the pressure between printheads and take into account the length of print runs or the prevalence of one color toner over another during a print run by changing the amount of ink entering specific zones of a print sheet. In addition, the offset printing paradigm requires long print runs to insure that set-up costs are recovered. This sometimes leads to excess production which in turn requires storage until demand returns. In general, under the offset printing paradigm, print runs are required to be of sufficient length to amortize costs of image assembly and platemaking over the long runs. The xerographic printing paradigm, on the other hand, is directed at short print runs because copies are printed on demand. Consequently, print data is easily varied (e.g., compressed) to reduce storage requirements.
Xerographic digital printing machines typically provide operators with the ability to adjust print appearance before the start of a print run. For example, color adjustments allow operators to optimize print appearance of output copies. However, once print appearance selections are made, they are applied to an entire print job before the print job is rendered to a binary representation for printing. Adjustments are typically made to print appearance when a raster is decomposed from a contone (continuous tone) image. A contone image is defined herein as a multilevel image that approximates a continuous tone image, and a raster image is defined herein as a binary image that represented by a plurality of printer dots. The 5775 Digital Color Copier, produced by the Xerox.RTM. Corporation, offers extensive control to an operator to optimize print appearance. Specifically, the 5775 offers control over copy output lightness and darkness, sharpness, contrast, and different document types. Lightness and darkness adjustments, for example, vary hue along a scale from black to white. In addition, operators can adjust the balance between colors and the overall amount of color on output copies. More specifically, an operator is able to adjust the amounts of the three process colors, Cyan, Magenta, and Yellow, (CMY) to shift color balance on copies to a preference.
Color printing systems such as the 5775 Digital Color Copier, however, do not provide real-time print appearance control. Real-time print appearance control is defined herein as dynamic control of print appearance while a copier is producing prints. Generally, real-time print appearance control is outside operator control since appearance control is invariant and maintained by embedded control systems. Existing print appearance control systems are static or unchanged while a print job or copy job is imaged on a photoreceptor. However, greater control by an operator of a printing system is desirable because print appearance is often subjective. It would therefore be desirable to provide an operator of a digital printing system with the ability to vary print appearance in real-time or during a print run. It would also be desirable that a user input device provide operator control near a printer output tray to insure real-time feedback of print output to the user during a copy or print run, or when proofing a job.
In accordance with the present invention there is provided, in a digital printing system with a controller for converging operating characteristics of a first process control module on a first operating point stored in a memory, an apparatus for adjusting print appearance of printer output in real-time, comprising: a process control sensor for providing feedback from adjustments made to the first process control module, said process control sensor providing an output signal; an input device for incrementally adjusting the first operating point, said input device overriding the first operating point with a second operating point; a comparator for determining a difference value between the output signal from said process control sensor and the second operating point, said controller adjusting the first process control module in accordance with the difference value determined by said comparator; an interaction look up table being stored in the memory; and means for adjusting a third operating point of a second control module, said adjusting means overriding the third operating point with a fourth operating point, so that adjustments made to the second process control module compensate for the adjustments made to the first process control module.
In accordance with another aspect of the present invention, there is provided an apparatus for providing real-time print appearance control to an operator of a digital printing system, comprising: a contone image; an image processing system for decomposing said contone image to a raster image in accordance with a set of image processing parameters; an input device connected to said digital printing system, said input device providing input signals to modify the set image processing parameters; and a controller for detecting input signals from said input device while the print request is printed by the digital printing machine, said controller invoking said image processing system to decompose the contone image in accordance with a modified set of image processing parameters to change the print appearance of the output of the digital printing machine.
One aspect of the invention is based on the recognition of a problem with conventional xerographic printing systems--they do not provide real-time print appearance control. This aspect is further based on the discovery that this problem can be solved by the addition of a user interface apparatus (e.g., a joystick) for the output appearance control. The apparatus described is advantageous because it adds operator flexibility to color printing processes similar to that found on lithographic presses. As a result of the invention, short-term xerographic process controls are overridden by joystick commands.